TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractHistorically, most UKCS production has been of light oil, 30 0
Summary In this paper we review progress made in developing United Kingdom Continental Shelf (UKCS) heavy oil fields. Reservoir productivity is compared with existing light oil developments and three categories of heavy oil reservoir are identified, which require the application of different well technologies to achieve acceptable offshore production rates. Case histories from existing developments and fields under appraisal are used to illustrate how advances in technology and effective risk management allow increasingly difficult heavy oil fields to be developed. Finally, the future direction for these heavy oil developments is discussed, looking at the scope for improved oil recovery (IOR) techniques and further technology developments to drive down costs and to increase reserves in fields currently under waterflood or to improve the economics of hitherto subeconomic fields. Introduction Early production from UKCS oil fields has been of light oil. However, a significant number of "heavy" (taken to refer to reservoirs with in-situ viscosities greater than 5 cp) oil fields have also been discovered. Most UKCS heavy oil is in relatively shallow reservoirs, comprising high porosity unconsolidated sands with excellent horizontal permeability (typically 3000 to 10 000 md) and very high vertical permeability (kV:kH) in the range of 0.2 to 1.0). The oil columns are usually at least partially underlain by water and some also have primary gas caps. This combination of reservoir parameters and the demanding offshore environment of the UKCS presents a special set of reservoir engineering challenges because of the difficulties in achieving and maintaining sufficiently high production rates to justify development. In this paper we provide an overview of the development of heavy oil fields on the UKCS, past, present and future, with an emphasis on the subsurface issues. This shows how the application of new technology, principally horizontal wells, extended reach drilling (ERD) and improvements in sand control has led to successful developments. Increasing confidence in this technology has allowed the Captain field (reservoir viscosity 88 cp) to be brought onto production and encouraged appraisal activity on other fields with viscosities as high as 1000 cp. It is conservatively estimated that there are around 10 billion STB of heavy oil in place on the UKCS. Less than a quarter of this resource is currently being developed. Assuming that recovery factors for the undeveloped stock tank oil initially in place (STOIIP) are likely to be in the range of 20 to 40% shows that there are approximately 1.5 to 3 billion barrels of additional reserves to be produced, which will make a significant contribution to the longevity of the UKCS. Heavy Oil Resources in the UKCS Many of the heavy oil accumulations discovered in the UKCS are in the northern North Sea, in the eastern margins of the East Shetland Platform. Other significant discoveries are in the Fladen Ground Spur, the Halibut Horst, and west of the Central Graben. Heavy oils have also been discovered in the Atlantic margin area. Fig. 1 shows the structural elements in the central and northern North Sea and the location of heavy oil fields under production or active appraisal. The majority of the discoveries are in Lower Tertiary sands and Fig. 2 shows the conceptual lithostratigraphy of the important reservoirs. The principal heavy oil reservoirs are in the Upper Palaeocene Maureen formation, the Heimdal sands in the Lista formation (e.g., Mariner), and the Dornach and Hermod sands in the Sele formation (e.g., Bressay), the Balder and Frigg sands (e.g., Gryphon and Harding) and the mid-Eocene Nauchlan sand (Alba). The Captain field, which was discovered in 1977, is in the Lower Cretaceous Captain sand, and has the lowest API oil and highest in-situ oil viscosity of any currently producing UKCS field.
Gas production from the UKCS commenced in 1967 and oil production in 1975. The North Sea area is now very much a mature province with the large fields in the Southern, Central and Northern North Sea producing at significantly below their early plateau production rates. Here the drive is to maximise the overall economic hydrocarbon recovery from the province, by making the best use of the infrastructure that has been built up to bring in new discoveries and improve recovery from the mature fields. New areas (deeper, harsher climate) are being opened up for exploration on the Atlantic Margin. This paper reviews the evolution of the mature areas of the UKCS, with case studies to illustrate the technical challenges that have been overcome. Over the years government and industry have expended considerable resources in developing innovative techniques for improved hydrocarbon recovery. These range from developments in the application of EOR processes to advances in drilling and reservoir management technology, including novel seismic techniques to identify new or bypassed oil. Technological advances have also unlocked reserves in heavy oils and in high-pressure high-temperature (HPHT) condensate fields, which were left undeveloped until the 1990s. Finally the potential for further exploitation and life extension of the UKCS as a significant hydrocarbon province will be reviewed. This will cover perceived technology gaps in opening up the new areas in deeper water, opportunities for redeveloping mature fields using new technology, combining IOR with carbon dioxide sequestration, and the need to drive down costs to be competitive in the international arena, while honouring environmental commitments. Introduction Oil exploration and production in the UK began onshore in the early part of the 20th century in the East Yorkshire, Lincolnshire and East Midlands areas. Later the interest extended to include the Dorset basin in the south of England. These were typically mechanical pump assisted fields producing a few 100 bbls/day/well. The first offshore gas field, West Sole in the Southern North Sea (SNS), was discovered in 1965 and brought onstream in 1967. Oil was first discovered in the Central North Sea (CNS) in 1969 and the first oilfield to come onstream was Argyll in 1975, followed soon after by the Forties field. The UK became self-sufficient in oil around 1980. Oil production on the UKCS has followed a typical exploitation path for a hydrocarbon producing area with large conventional fields being developed first and thereafter smaller fields utilising the infrastructure. It has now entered a third phase with the development of technically more difficult fields such as heavy oil fields and High Pressure High Temperature (HPHT) fields. A recent paper1 reviewed the UKCS heavy oil fields, so the primary focus of this paper is light oil fields. In the early days the industry used the term Enhanced Oil Recovery (EOR) to describe the deliberate injection of an alternative fluid to displace further oil from reservoir rock, over and above the standard pressure maintenance strategy (waterflooding for most UKCS oil fields). In the 1990s the industry began to use the term Improved Oil Recovery (IOR) to cover any operation (including the EOR techniques) that increased oil recovery above the figure that had been initially accepted as economically and technically exploitable. ‘Improved Hydrocarbon Recovery’ is a more general term that will cover all hydrocarbon types, including gas, but it is not commonly used as an acronym.
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